Window Boxes

Window boxes allow us to brighten up even the smallest area with seasonal colour and foliage.

Window boxes are often at their best when overflowing with trailing flowers such as lobelia, verbena and cascading pelargoniums. However, window boxes can also be used to create a mini-wildlife border that attracts butterflies and bees, a small herb border, or a mini fruit and vegetable bed.

Safety

Whatever plants you choose for your window box, make sure that the box itself is secure on your window sill. If the sill slopes downwards, place two or three wooden wedges the front of the box, to level it.

To be absolutely sure that the box is safe you should fix brackets to the wall on either side and one underneath the sill. Each bracket should protrude across the front of the box to prevent it slipping.

If your windows open outwards, you will need to fix the window box below the window sill. Fix it at least 30cm/12in lower that the sill, so that tall plants will keep their heads when the window is opened.

Materials

Window boxes are available in a range of sizes and different materials.

Terracotta boxes look very attractive but you must be sure that they are frost-proof if winters are severe in your area. They lose water very quickly, so you must be prepared to check regularly that the soil hasn’t dried out: in hot weather this may mean twice a day. They are relatively expensive and can be heavy, making them unsuitable for some windowsills.

Wooden boxes need to be treated against rot. Choose a wood preservative that will not be harmful to the plants in the box. As an extra precaution, line the sides of the box with plastic to prevent water getting into the wood from the inside.

Plastic window boxes are probably the best buy for all-year use. They are waterproof and don’t crack in the sunshine or in frost. Again, it’s important to keep an eye on the plants and water them frequently. Drainage holes are usually marked on the base of the box, but you will have to open them using a hand drill. Drip trays are usually bought separately. They hold excess moisture and stop water draining down over your window ledge.

Maintenance

To keep your window boxes looking their best, regular maintenance is important. To give your plants the best start, use a loamy, soil-based compost such as John Innes No. 2 or a special water-retentive compost. Water the box with a liquid fertiliser every fortnight during the growing season.

When preparing your box, place a layer of stones or terracotta crocks in the base to improve drainage. The deeper your box, the taller and larger the plants you can grow in it. For best effects, and if your window sill is large enough, a box 90cm long, 22cm deep and 25cm wide is a good size to work with.

Possible problems:

Brown or shrivelled leaves are usually a sign of wind burn or lack of water. In winter, water very sparingly, as the soil will not dry out so quickly.

Aphids are a common window box problem. Spray the plants with soapy water from time to time when you see the first signs of window damage.

Themed Window Boxes

Herb box: (left) Upright rosemary, sage, thyme and basil offer a good range of colours, shapes and flowers, and are among the most useful fresh herbs. If the window box is kept on your kitchen window sill, you can snip herbs quickly as you need them for cooking.

Salad box: ‘Tom Thumb’ lettuce, clumps of chives and ‘Pixie’ tomatoes make a good mix or try sow cut-and-come-again lettuce. There are some nice loose-leaf varieties available now, including red, oak leaf and peppery leaves ideal for sow-and-cut.

Spring box: PlantAlyssum saxatile in front to trail over the front of the box; grape hyacinth bulbs in the next row and dwarf tulips such as Tulipa greigii hybrids or a small Narcissus variety at the back of the window box.

Cascade box: A mixture of trailing fuchsias, ivy-leaved pelargoniums, variegated ivy, verbena and trailing lobelia can make a striking mix.

Up-and-down box: For a cascade combined with an upright effect, try ‘Knee-Hi’ sweet peas at the back of the box with trailing ivy-leaved pekargoniums in the middle of the box. For trailing foliage add ivy.